Technical Field
The present invention relates to an engine starting apparatus capable of switching an output characteristic of a starter between a low-torque high-speed characteristic and a high-torque low-speed characteristic.
Related Art
Conventionally, there is a desire for an idle-stop-enabled starter to reduce an engine restarting time as much as possible to improve the comfort of a driver or other occupants of a vehicle. A technique for reducing the restarting time includes increasing a cranking speed. Such a technique, however, in combination with good start-up performance at low temperatures where engine friction is high, necessitates use of a large in size and high power motor.
A known technique for increasing the cranking speed at restarting of the engine without increasing dimensions of the motor includes switching as needed between high-speed and high-torque characteristics.
For example, Japanese Patent Application Laid-Open Publication No. 2004-197719 discloses preparing a motor including a series coil and a shunt coil as field coils and switching between the high-speed and high-torque characteristics by an electronic control unit (ECU) controlling field current flowing through the shunt coil of the motor.
The technique disclosed in Japanese patent application laid-open publication no. 2004-197719 can reduce an engine starting time from engine idle stop by setting an output characteristic of the starter at starting of an engine to a high-speed characteristic. At the high-speed characteristic of the starter, however, a rotational speed of a pinion may become too high for the pinion to reliably mesh with a ring gear, which may lead to diminished engagement reliability. A measure to overcome such a disadvantage may include increasing a load of a drive spring for pushing the pinion toward the ring gear at mesh.
Increasing the load of the drive spring, however, necessitates increasing an attraction force of a solenoid for attracting the pinion thereinto against a spring reaction force generated by the drive spring. This may lead to larger dimensions of the solenoid, thus leading to degradation of installation property of the starter and an increased starter cost.
In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing an engine starting apparatus capable of reducing an engine starting time without increasing the size of the solenoid.